The present invention relates to an electromagnetic friction clutch having at least two clutch parts that are mounted so as to be rotatable relative to each other and may be pressed against each other by the magnetic force of a magnetic circuit guided in the clutch parts in a soft magnetic material, at least one electromagnet being situated in the magnetic circuit to change the magnetic flux in the clutch parts.
A friction clutch of this sort for a lawnmower is known from European Patent Document 0 537 022 B1. The clutch has a first clutch part, configured in a cup-shaped manner and positioned on a drive shaft of a motor, and a second clutch part, parallel to the floor of the cup-shaped clutch part and having an approximately disk-shaped configuration. The second clutch part is joined by leaf springs in a rotationally fixed manner with a belt pulley that has a drive connection with a cutting blade, the belt pulley being also positioned on the drive shaft, and being rotatable relative to the drive shaft via a bearing. The second clutch part is movable axially to the drive shaft relative to the first clutch part and may be moved into an engaged position, in which it is in contact with the first clutch part in a frictionally engaged manner, and into a disengaged position in which it is offset from the first clutch part. To engage the second clutch part, there is an electromagnet, which has a field housing and a coil for producing a magnetic flux. The field housing is part of a magnetic circuit passing through the clutch parts and running in a soft magnetic material and across air gaps. The magnetic circuit is guided into the clutch parts in such a way that the magnetic flux changes between the clutch parts at six flux crossover points located one after another in the direction of flux. The friction clutch has the disadvantage that the time for switching the friction clutch between the engaged (“adhering”) and the disengaged (“slipping”) positions is still relatively long. The friction clutch is therefore not suitable in practice for applications in which quick engagement and disengagement are necessary. Since the clutch parts that are rotatable relative to each other do not have a defined rotational position relative to one another, the clutch is also not usable for applications in which a predefined angle of rotation between the clutch parts must be set.